The present invention relates to a method for the preparation of a rare earth-based permanent magnet having outstandingly high magnetic anisotropy or, more particularly, to a method for the preparation of a rare earth-based permanent magnet of the neodymium-iron-boron type having outstandingly high magnetic anisotropy.
As is well known, a a method for the preparation of a rare earth-based permanent magnet of the neodymium-iron-boron type, referred to as a Nd--Fe--B magnet hereinafter, has very excellent magnetic properties and the material cost for the preparation thereof is remarkably low as compared with the samarium-cobalt magnets so that consumption of Nd--Fe--B magnets is rapidly increasing.
Like rare earth based magnet alloys of other types, the method for the preparation of a Nd--Fe--B magnet alloy includes the melting method and the quenching method, of which the former method is more widely practiced in the magnet industry than the latter method. In the melting method, high purity metallic or elemental materials of neodymium, iron and boron are taken in such a proportion as to correspond to the alloy composition of Nd.sub.2 Fe.sub.14 B or somewhat richer than that relative to the amounts of neodymium and boron and they are melted together to form an alloy melt which is cast to form an ingot to be pulverized into fine particles having an average particle diameter of a few micrometers.
The fine particles of the rare earth based magnet alloy are then shaped by compression molding in a magnetic field to give a green body or powder compact which is subjected to a heat treatment first to effect sintering at a temperature of about 1100.degree. C. and then for aging at a lower temperature to give a sintered rare earth magnet having magnetic anisotropy. The above mentioned compression molding of the alloy particles in a magnetic field is effective to accomplish alignment of the alloy particles relative to the axis of easy magnetization of the individual particles so as to impart the sintered magnet with magnetic anisotropy reported in, for example, Japanese Journal of Applied Physics, volume 26, 1987, page 785, by M. Sagawa, et al.
The above mentioned quenching method is also called the meltspun method, according to which a melt of the magnet alloy is ejected from an orifice at the surface of a quenching drum rotating at a high speed so that the alloy melt is instantaneously solidified in the form of a thin ribbon which is amorphous or has relatively low crystallinity (see, for example, Physics Letters, volume 46, 1985, page 790 by R. W. Lee, and other reports). The thus obtained thin ribbon of the magnet alloy by quenching is processed into a magnet body in several different ways including the processes called MQ1, MQ2 and MQ3.
In the MQ1 process, the quenched thin ribbons of the magnet alloy are pulverized into a fine powder which is blended with a resinous material as a binder and the blend is shaped into the form of a magnet body which is a kind of so-called bond magnets. Though advantageous in respect of the low costs for magnet production, the MQ1 process is disadvantageous due to the relatively low magnetic properties of the bond magnet which is not magnetically anisotropic but isotropic because no measure is undertaken for the alignment of the magnet particles relative to the axis of easy magnetization in addition to the low packing density of the magnet particles in the blend. In the MQ2 process, the thin ribbons obtained by quenching of the alloy melt are finely pulverized and shaped as such in a hot press into the form of a magnet body which is a magnetically isotropic bulk magnet. In the MQ3 process, the MQ2 magnet is further subjected to a plastic working at a high temperature so as to accomplish alignment of the axis of easy magnetization in the direction of compression. A further processing of the MQ3 magnet is reported, in which an MQ3 magnet is pulverized and the powder is blended with a binder resin to give a blend which is shaped by compression molding in a magnetic field so as to give a magnetically anisotropic bond magnet although this process is not industrialized because of the high costs for the process involving very complicated steps.
As compared with the anisotropic bulk magnets of Nd--Fe--B magnet alloys, bond magnets are mostly isotropic as is the case with the MQ1 magnets and anisotropic bond magnets are still at a beginning stage of development presumably due to the fact that a great decrease is caused in the coercive force of the magnet when an ingot of the Nd--Fe--B magnet alloy or a sintered magnet is finely pulverized.
In view of the above described problems, a method has been developed and is reported by T. Takesita, et al. in Proceedings of 10th International Workshop on Rare Earth Magnets and Their Applications, Tokyo, 1989, page 339, in which the Nd.sub.2 Fe.sub.14 B alloy is subjected to a heat treatment in an atmosphere of hydrogen gas to give an agglomerate of fine crystallites of the alloy. This process comprises the successive steps of hydrogenation, disproportionation, desorption and recombination and sometimes called the HDDR process. It is further known that the particles obtained therein can be imparted with magnetic anisotropy when the alloy is admixed with certain adjuvant elements such as cobalt, gallium, zirconium, hafnium and the like so as to enable preparation of an anisotropic bond magnet of the Nd--Fe--B alloy. This method, which is relatively simple, is now under active investigations although several problems and disadvantages are involved therein that the crystallite diameter therein is larger by almost one order than in the quenching method to cause a difficulty in the application of the method to the nanocomposite materials discussed later, that a great danger against the workers' safety is unavoidable in the heat treatment in an atmosphere of hydrogen gas and that the desirable magnetic anisotropy cannot be obtained without addition of the adjuvant elements.
As an outcome of the development works for a rare earth based permanent magnet of the next generation having further improved magnetic properties, the so-called nanocomposite magnet materials are now highlighted as is reported in IEEE Transaction Magnetics, volume 27, 1991, page 3858 by E. F. Kneller, et al. and elsewhere. The nanocomposite magnet material is a two-phase composite dispersion system consisting of a hard magnetic phase and a soft magnetic phase each finely dispersed in the other with a fineness of the order of several tens of nanometers. By virtue of the magnetization coupling of both phases with the exchange interaction, the soft magnetic phase in the composite is strongly inhibited from inversion of magnetization so that the composite body as a whole exhibits a behavior like a single hard magnetic phase to give a possibility of obtaining a further increased saturation magnetization without causing a decrease in the coercive force. According to the calculation reported by R. Skomski, et al. in Physical Review, volume B48, 1993, page 15812, a maximum energy product (BH).sub.max of as high as 137 MGOe could be obtained in the Sm.sub.2 Co.sub.17 N.sub.3 /Fe-Co system assuming accomplishment of magnetic anisotropy therein.
The nanocomposite magnet systems heretofore investigated include the system of Nd.sub.2 Fe.sub.14 B/Fe.sub.3 N reported by R. Coehorn in Journal de Physique, volume 49, 1988, page C8-669, Nd.sub.2 Fe.sub.14 B/Fe disclosed in Japanese Patent Kokai 7-173501 and 7-176417 and reported by L. Withanawasam, et al. in Journal of Applied Physics, volume 76, 1994, page 7065, Sm.sub.2 Co.sub.17 N.sub.3 /Fe reported by J. Ding, et al. in Journal of Magnetism and Magnetic Materials, volume 124, 1993, page L1, and others. As a method for the preparation of the two-phase composite systems in the nanocomposite materials disclosed there, a thin ribbon or powder of an amorphous alloy obtained by the melt-spun method or mechanical alloying method is subjected to a heat treatment to cause formation of microcrystallites.
Because of the non-alignment of the particles relative to the axis of easy magnetization, the nanocomposite magnets obtained heretofore are limited to isotropic magnets which are inferior to the magnetically anisotropic magnets in the magnetic properties.